Abstract: A fluid pressure system and stoplight control circuit for a towed vehicle which is either an intermediate unit or the last unit of a vehicle train includes an inlet supply line for receiving fluid pressure from a preceding unit and an outlet supply line which is connected to a corresponding coupling of a succeeding unit when the vehicle is an intermediate unit and which is connected to a coupling hanger when the unit is the last unit of a vehicle train. A pressure responsive stop light switch is connected into the coupling hanger. Accordingly, when the outlet supply line is connected to the coupling hanger, the contacts of the stop light switch are closed, thus permitting power to the stop lights when a brake application is effected. However, when the outlet supply line is connected to a succeeding vehicle, the coupling hanger is vented, so that the contacts of the pressure responsive switch are open, thus interrupting power to the stop lights and conserving power on the vehicle.

Abstract: A fluid pressure braking system for a tractor trailer vehicle includes a supply line connecting the tractor and trailer braking systems. A low pressure responsive switch responds to the pressure level in the supply line for actuating a warning device when the pressure level in the supply line drops below a predetermined pressure level to thereby warn the driver before a brake actuation is effected that a low pressure level exists in the trailer braking system. A normally open stoplight switch is connected in series with the low-pressure responsive switch so that the signal generated by the low-pressure responsive switch is prevented from actuating the warning device when the tractor is operated in the "bobtail" mode without a trailer.

Abstract: A limited service application of the service brakes of the trailer portion of a tractor trailer combination vehicle is applied and held while the vehicle operator gets out of the cab to make a pre-trip inspection. The service actuation is initiated by releasing the trailer parking brakes while the tractor parking brakes remain applied and then making a momentary service brake actuation by use of the vehicle dual control valve. A control valve assembly responds to the service actuation to hold a service actuation of the trailer brakes until either the tractor parking brakes are released or the trailer parking brakes are applied.

Abstract: An air dryer control system is disclosed in which the compressed air output of a compressor is dried by one of two air dryers connected in parallel. The purging and drying cycles of the air dryers are alternated by timing and relay device. Upon start-up, the output of the compressor is dried by both air dryers until the pressure level in the supply reservoir attains a predetermined level. When the governor cutout pressure is achieved in the reservoir, both air dryers are purged.

Abstract: A fluid pressure braking system for a vehicle having service brakes and spring activated parking brakes includes electropneumatic activators for effecting and holding a limited service brake application while the driver leaves the vehicle to make a route pick-up. Accordingly, the spring brakes are not used during route pick-ups, thereby increasing the life of the spring actuator since it is not repeatedly cycled. An electropneumatic interlock automatically effects a parking brake application if the vehicle ignition is switched off while an electropneumatic service actuation is effected.

Abstract: A suspension control system for controlling a liftable axle of a vehicle between a retracted, non-load bearing position and an extended, load bearing position includes a valve mechanism which responds to the pressure level in the air bags of the air suspension system of the non-liftable rear axle of the vehicle. The system includes a leveling valve which increases pressure in the air bags to maintain a constant vehicle height. Accordingly, the pressure level in the air bags in directly proportional to vehicle loading. The valve means responds to this pressure level to communicate pressure to the actuator means controlling the liftable axle when the pressure level in the air bags on the non-liftable rear axle attains a predetermined level. The liftable axle responds to this increase in pressure to extend into load bearing engagement with the road.